feat: support null-friendly parameters

[vishalg0wda]

This PR enables the nullFriendlyParameters feature in Java SDK generation, significantly simplifying how SDK end users provide optional and nullable values.

💡 What’s Changing
Prior to this change, end users had to wrap values explicitly:

.setMyParameter(Optional.empty());
.setAnotherParameter(JsonNullable.of(null));

With this feature enabled, users can now pass null directly:

.setMyParameter(null);
.setAnotherParameter(null);

This improves:

Developer experience – Cleaner, more idiomatic Java usage.

IDE feedback – JetBrains users get nullability hints via jakarta @Nullable / @NonNull annotations.

API cleanliness – Wrapper types like JsonNullable are handled internally and no longer leak into user code.

⚠️ Breaking Change — Rollout Considerations
This introduces breaking changes to method signatures and generated types. Any SDKs regenerating with this flag enabled will break compatibility for existing consumers.

As such:

This change must be released behind a version bump (typically a major version).

🤔 PATCH Semantics and Null Handling
A common concern with this change is how null values behave in PATCH-like operations, where setting a field to null often carries different intent than omitting it entirely.

To clarify:

✅ Nullable fields → passing null results in an explicit null being sent.

✅ Optional fields → passing null causes the field to be omitted from the request.

✅ Nullable + Optional fields → passing null results in null being sent; to omit, simply avoid calling the setter.

The generated SDK code handles this distinction under the hood, by converting values internally to the appropriate JsonNullable or JsonUndefined types - so users don’t have to manage those concerns themselves.

[Amzani]

Thanks @vishalg0wda for the PR, I’ll first need to assess the impact of this change on our existing customers before including it in the upcoming release.

Regarding the PR itself, it should only include changes to the gen.yaml file, since the actual SDK generation is managed by our CI pipeline.